Proceedings of 10th International Conference on Nuclear Physics at Storage Rings (STORI
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17)

Proc. 10th Int. Conf. Nuclear Physics at Storage Rings (STORI’17)
JPS Conf. Proc. 35, 011006 (2021)
https://doi.org/10.7566/JPSCP.35.011006

Status of the Laser Spectroscopy and Merged-beam
Experiments at RICE
Y. Nakano1,2 , R Igosawa3 , S Iida1 , S Okada2 , M Lindley2 , S Menk1,2,4 , R. Nagaoka1 ,T. Hashimoto5 , S. Yamada4 , T. Yamaguchi3 , S. Kuma2 , T Azuma2,41

Department of Physics, Rikkyo University, Tokyo 171-8501, JapanAMO Physics Laboratory, RIKEN, Saitama 351-0198, Japan3
Department of Physics, Saitama University, Saitama 338-8570, Japan
4
Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397, Japan
5
Japan Atomic Energy Agency (JAEA), Tokai 319-1184, Japan2

E-mail: nakano@rikkyo.ac.jp(Received February 28, 2018)
Recently, we reported the commissioning of the new cryogenic ion storage ring RICE, which demonstrated potential capabilities for the precise studies of molecular structures and reaction dynamics. In
the present article, we describe the status of experimental programs ongoing at RICE with a focus on
the laser spectroscopy and merged-beam collision experiments.
KEYWORDS: Electrostatic storage ring, laser spectroscopy, neutral beam,
TES microcalorimeter

1.

Introduction

The RIKEN Cryogenic Electrostatic ion storage ring (RICE) has been built, and its operation
has been tested at cryogenic temperatures down to 4.2 K [1]. The commissioning experiments with a
15 keV-Ne+ ion beam demonstrated a storage lifetime of ∼ 780 s, and the residual gas density was estimated at a few 104 cm−3 . In addition to providing such an extreme vacuum condition, the cryogenic
cooling of the ring opens a new opportunity to study the molecular structure and reaction dynamics
in the absence of excitation by blackbody radiations. Radiative cooling of the molecular ions into therovibrational ground state during storage have already been demonstrated in the cryogenic storagerings DESIREE [2] and CSR [3, 4].
With the aid of the new capabilities provided by the cryogenic storage ring, several experimental
programs are currently running at RICE. Among all the experiments that are going to be performed
at RICE, probing the internal temperature of stored molecules is one of the most important tasks.
Therefore, a dissociation spectroscopy of molecular ions is currently being performed for the direct
and real-time observation of the rovibrational population of molecules during storage. In addition, toprepare for injections of pre-cooled molecular ion beams, a cryogenic ion trap [5] and a helium droplet
source [6] are being developed in parallel. The ion trap injection beamline is currently equipped with
an electrospray ionization source for large molecular ions and a laser ablation source for clusters ions.
Ions from either source were successfully accumulated in the cryogenic linear octupole trap, and thenextracted and accelerated to ∼10 keV for injection. The ion trap beamline is already connected to themain beamline of the RICE via a quadrupole deflector, and ion injection is under commissioning.
Another important part of the facility is the laser and neutral beam sources. As shown in Fig. 1(a),the laser and neutral beams will be introduced from Port 3 to be merged with the stored ion beamat the straight section. Tunable laser systems, visible-UV OPO, IR OPO, and dye lasers are installed
together with necessary control and detection systems for the planned laser experiments at RICE.
1■■■011006-1©2021 The Author(s)This article is published by the Physical Society of Japan under the terms of the Creative Commons Attribution 4.0 License. Any further distribution of this work
must maintain attribution to the author(s) and the title of the article, journal citation, and DOI.

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JPS Conf. Proc. 35, 011006 (2021)(a)Cold molecular beam
(Ion trap / He droplet)

MovableMCP detector
Neutral Port 4
detector

Port 3

Port 1

Port 2MagnetCurrent / uA

ECR

140
120

100 W

(b)

50 W
10 W

80
60

5W

N

+

N

+
N2+ NO

2+

40
20

O2+

0
0

Current / uA

Neutral beam

100

140
120

Laser

O+

10

N2O+

O 2+
20

30
m/q

40

(c)

50
5.0E-05 mbar

1.0E-04 mbar

100

1.0E-03 mbar

80
60

40
20

0
0

10

20

30
m/q

40

50

Fig. 1. (a) Schematic view of the RICE beam line configuration. Laser and/or neutral beams are injectedfrom Port 3, and the neutral particle detector is installed at Port 4. (b) Mass spectrum obtained from the ECR
ion source operated at diﬀerent microwave powers, namely, 5, 10, 50, and 100 W. The source gas was N2 O
and the source pressure was 5.0×10−5 mbar. (c) The same but measured at diﬀerent source pressures. The
microwave power was kept at 5 W.

The neutral beamline has already been constructed, and it is under oﬀ-line testing with negative
ion sources and a photo-detachment laser at Rikkyo University. In addition, the development of anenergy-sensitive neutral detector has been started. In the following, the current status of these experiments and developments will be reported.

2.

Laser dissociation experiments

2.1

Storage of molecular ions
Molecular ions were produced by an ECR ion source described in Ref. [1]. The ECR ion source
is designed for high current beams of multiply charged atomic ions, and is generally operated witha high microwave power for maintaining a stable plasma. However, running the source with a low
microwave power enables molecular ions to remain in the source plasma without being broken up.
Figure 1(b) shows the mass spectrum from the ECR source operated with 5×10−5 mbar N2 O gas underdiﬀerent values of microwave power, namely 5, 10, 50, and 100 W. Under the high microwave input,
the mass corresponding to N2 O+ was not observed but its fragments dominated the mass population.
When the input microwave power was reduced to 10 and 5 W, the intensity of the fragment ionsdecreased while the N2 O+ beam current increased. The source pressure dependence of the mass2■■■

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Fig. 2. Control and timing chart of the laser and data taking system.

spectra under a low microwave power of 5 W is shown in 1(c). The intensity of the N2 O+ beam wasenhanced by increasing the N2 O gas flow into the source. Under a source pressure of 1 × 10−3 mbar,
the N2 O+ beam current was around 70 µA. After passing through a 2 × 2 mm slit and following beamtransport sections, the beam current at the injection was 10 nA.
2.2

Laser setup
A narrow-band pulsed dye laser was installed at Port 3 of the storage ring (see Fig. 1). For the
dissociation spectroscopy of N2 O+ ions at a laser wavelength of around 320 nm, the second harmonics
from the 532 nm pumped DCM/Ethanol dye laser output is used. The pumping source was recently
upgraded from a flashlamp-pumped 10 Hz Nd:YAG laser to a high-repetition DPSS Nd:YAG laser
with a pulse energy of 80 mJ at 100 Hz. The typical pulse energy at the output of the second harmonics
generator is 5 mJ.
Fig. 2 shows the control and timing sequence of the storage ring and laser system. The mastertrigger for the ring injection cycle and external Q-switch trigger for the laser were generated by a
two-channel function generator in a synchronized mode. While the laser is constantly triggered at
100 Hz, the repetition cycle of the ring master trigger depends on the experiments, e.g., more thanthousands seconds for long-period storage measurements. The laser light is introduced into the ring
through a mechanical shutter, which opens at a certain delay time after the ion injection into the ring.
The logical conjugation of the laser trigger and shutter signal was sent to the DAQ PC as the signalof the laser irradiation into the ring.
2.3

Neutral particle detector
In the laser dissociation experiment, the laser beam is merged with the stored ion beam at thestraight section of the ring. Laser-induced dissociation of molecular ions such as
AB+ + hν → A+ + B
3
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(1)

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Laser

(a)

Neutral

100
MCP

10

Count

CEM

e-

(b)

Mirriordynode

CEM

1
0.1
0.01
0.001
2

Laser

1

10

(c)

4

6

8

0.001

2

4

6

8

0.01
Time /sec

2

4

6

8

0.1

Laser-induced
neutral fragments

0

10

-1

Count

10

Background neutrals

-2

10

-3

10

-4

10

0

100

200

300
Time / sec

400

500

600x10-6

Fig. 3. (a) A three-dimensional view of the neutral particle detector. (b) Decay curve of the 15 keV N2 O+
ion beam in the RICE observed by the movable MCP detector (red) and the CEM of the mirror dynode detector(blue). The vertical axis represents the normalized count per TDC channel with a 167 µs bin size. (c) The samemeasurement by the mirror dynode detector but shown in a diﬀerent bin size of 655 ns. A pulsed laser was
introduced at 250 µs after the injection.

can be identified by detecting the neutral fragment that flies straight to the outside of the ring throughPort 4. Microchannel plates (MCP) are widely used as detectors for keV neutral particles in ion beam
experiments. However, the direct UV laser irradiation will immediately lead to an irreversible damageon the MCP surface. In this respect, MCPs are not suitable for the collinear laser experiments, andtherefore, a dedicated detection system has been developed. The detection method is based on theconversion of a neutral particle into secondary electrons on a dynode surface. As shown in Fig. 3(a),the laser and neutral particles impinge at 45 degrees on the conversion dynode, which is a purealuminum plate finely polished to a mirror surface. Because aluminum has a high reflectance in the
UV region, more than 90 % of the laser light is reflected to the outside of the chamber through the
fused silica window. On the other hand, hits of the fast neutral particles produce secondary electrons
from the dynode. These electrons are then accelerated towards the channel electron multiplier (CEM)
installed in front of the dynode.
The mirrored dynode was installed at Port 4 of the RICE, ∼450 mm behind the movable MCP
detector, and its eﬃciency was evaluated without lasers. Figure 3(b) shows the decay curves of the
15-keV N2 O+ ion beam measured by the MCP and CEM of the mirror dynode detectors. Because
the dark count rate of CEM is lower than that of the MCP, the CEM measurement showed a decay
curve with a better signal-to-noise ratio. Considering the fact that the opening ratio of the MCP is4■■■

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Cs sputter
source17)

DuoplasmatronsourceMagnetMagnetPhoto-detachment
chamberSlits andFaraday cup

HV platform

Fig. 4. Photographs of the test beam line for the Cs sputter (left), and the duoplasmatron (right) ion sources.

around 60 %, the mirror dynode detector is expected to have a higher detection eﬃciency than the
MCP. The collection eﬃciency of the secondary electrons to the CEM is presently under the processof improvement.
The response of the mirror dynode detector to the laser shot was tested with the N2 O+ ion beam
the and the 323 nm dye laser pulse. The time spectrum of the CEM signal is shown in Fig. 3(c), where
the time origin corresponds to the beam injection. Each bunch of the neutral signal corresponds to thecirculation of the ion bunch in the ring. The laser was fired 250 µs after the injection, which created a
sharp peak on the spectrum by the scattered laser light and photoelectrons from the dynode. Around
10 µs after the laser was fired, the neutral yield was enhanced by two orders of magnitude due to thelaser-induced dissociation of the N2 O+ ions. This delay corresponds fo the time of flight of neutral
particles from the reaction region to the detector. Counting the number of laser-induced signals as afunction of the laser wavelength enables the in-ring dissociation spectroscopy of molecular ions.

3.

Neutral beam injection

3.1

Negative ion source
The neutral beam is generated by a laser-induced electron detachment of a negative ion beam [7].
Two diﬀerent ion sources, the duoplasmatron and the cesium sputter ion sources, were installed on thebeamline at Rikkyo University for the oﬀ-line testing (Fig. 4). The cesium sputter ion source (SNICS
II, National Electronics Corp.) was prepared for solid targets, such as carbon, silicon, and sulfur, andtested with 12 C at diﬀerent extraction energies ranging from 5 to 20 kV. By using a set of beam profile
monitors (BPM80, National Electronics Corp.), the source operation and beam transport parameters
were optimized. An additional beam steerer was installed after the dipole magnet to compensate
for the misalignment of the source and the magnet. At present, several microamp of C− beam istransported to the end of the test beamline.
For gaseous targets like hydrogen, deutrium, and oxygen, a duoplasmatron ion source (High
Voltage Engineering Model 358) was also set up as an injector. The source has been installed on a
30 kV high-voltage platform with the driving power supplies controllable via a wireless network,
and it is connected to a 100-mT dipole magnet followed by an XY slit and a Faraday cup for mass
analysis. Following the computer simulations of the beam trajectory, the source operation is currently
under testing using noble gases.
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(a)

(b)

slow axis

fast axis

Fig. 5. (a) Example of the laser ray tracing simulation using Zemax. The upper and lower figures show theresults for the fast and slow axes, respectively. (b) A photograph of the laser test bench.

3.2

Photo-detachment laser
An 808 nm high-power diode laser array is used to neutralize the negative ion beam by photodetachment. The laser head is a stack of 60 diode bars with a continuous output power of 5 kW in
total. A 15 kW DC power supply and deionized water circulating system with a cooling power of
7 kW were installed to drive the laser. The power supply is securely interlocked by water flow andtemperature monitors. The output power of the laser was measured using a plano-convex lens anda high-power laser probe (FLASH-6K, GENTEC) as a function of the electric current of the power
supply. The laser threshold current was ∼18 A, and the operating current at 5 kW was ∼92 A.
The diode array has a large emission area of 100 mm × 12 mm and a large angular divergence of∼40 and ∼5 degrees in the fast and slow axes, respectively. With the use of a microlens array, the fastaxis divergence was reduced to less than 0.5 degree. However, a well-designed optical system is stillneeded to increase the eﬃciency of photo-detachment. We carried out a laser ray-tracing simulation
using Zemax Optic Studio. Figure 5(a) shows an example of the simulated laser ray in the fast andslow axes. The beam width is converged to 1.2 mm by using three convex cylindrical lenses. The
real optics are constructed on a laser test bench (Fig. 5(b)) with multiple ND filters and a laser beamprofiler. A multi-path amplification of the laser light in a cavity is under preparation.

4.

Development of energy-sensitive neutral particle detector

4.1

TES microcalorimeters for mass spectrometric identification of neutral molecules
Identification of molecular products in the merged-beam experiment is important for achieving acomprehensive understanding of the reaction mechanism. However, neither the MCP nor the mirror
dynode detector described above has suﬃcient energy resolution to measure the energy deposit forthe mass spectrometric identification.
The Transition-Edge-Sensor (TES) microcalorimeter is a cryogenic detector that takes advantage
of superconductivity [8]. TES measures the tiny temperature rise resulting from the interaction withan incident particle due the abrupt increase in the resistance of a thin superconducting film that isbiased within a sharp transition edge between the normal and superconducting phases. This results ina high energy resolution. Though being developed for photon detection, e.g. for X-rays [9], TES has
suﬃcient sensitivity for any particle whose energy can be converted into heat. Here, we aim to apply
this detector to the low-energy (∼ 10 keV) molecule measurement at RICE.
4.2

Energy resolution of the TES microcalorimeter
The TES is normally operated at 100 mK being shielded from heat radiation using infrared blocking filters made of thin aluminum foils at each of the three cooling stages, 50 mK, 3 K, and 50 K [10].
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FWHM energy resolution [eV]

17)

12
10

Open

8
6
4

Close
0
0.2 0.4 0.6 0.8 1.0
Aperture ratio of 50 mK shieldFig. 6. FWHM energy resolution of the TES spectrometer for 6-keV X-ray as a function of the aperture ratioof the 50 mK shield. The error bar shows the RMS of the dispersion for each TES pixel.

Unlike X-rays, the low-energy molecules cannot pass through the filters made of 100-nm-thick aluminum foil. Here, it should be noted that the two shields at 3 K and 50 K are fortunately avoidable in
RICE because it is cooled already at 4 K.
For this application, the eﬀect of the 50 mK shield against the radiation background was evaluated from the Mn Kα (∼ 6 keV) spectrum obtained with a 55 Fe radioactive source for diﬀerent
configurations of the shield. The detector system is based on a 240-pixel TES array developed by
NIST (the same system as that in Ref. [10]), where only a part of the pixels is used. Under normal
conditions, with a 50 mK shield having a 5-µm-thick aluminum foil, the FWHM energy resolution ofa summed spectrum of 23 working TES pixels was 4.7 eV. When the shield is completely removed(being fully open to the 3K radiation), the TES system still functions with an energy resolution of 11.3
eV. Figure 6 shows those results together with results of the measurements with copper foils having amesh structure with diﬀerent aperture ratios. In summary, the condition can be chosen to optimize a
trade-oﬀ between energy resolution and aperture ratio (eﬃciency for the incident particle). Following
this study, we are preparing an actual setup for molecular-beam measurement in RICE towards thepractical use of the merged-beam experiment.

5.

Acknowledgement

This work was supported by the RIKEN Basic Science Interdisciplinary Research Projects, andby the JSPS KAKENHI Grant Number 26220607. Y.N. acknowledges the financial support from
JSPS KAKENHI (No. 16K13859 and 17H02993). S.K. and S.O. thank the members of the HEATES
collaboration for their support and discussions towards this challenging application of TES to the lowenergy molecule measurement, and acknowledge the financial support from Yamada Science Foundation and JSPS KAKENHI (Nos. 26707014, 16H02190). S.Y. acknowledges the financial support from
JSPS KAKENHI (No.15H05438). T.H acknowledges the financial support from JSPS KAKENHI(No.16K17718). This work was partially supported by MEXT-Supported Program for the Strategic
Research Foundation at Private Universities, 2014-2017 (S1411024).
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